Fluorinated Elastomers
Fluorinated polymers enjoy widespread use as hydrophobic, oleophobic coatings. These materials exhibit excellent environmental stability, high hydrophobicity, low surface energy and a low coefficient of friction, and are used in a number of applications ranging from non-stick frying pans to optical fiber cladding.
Most fluoropolymers, however, are plastics that are difficult to process, difficult to apply and are unsuitable as coatings for flexible substrates due to their high rigidity. One example of a widely used fluorinated material is Teflon, a polytetrafluoroethylene. Teflon is difficult to process in that it is a rigid solid which must be sintered and machined into its final configuration. Commercial application of Teflon as a coating is complicated by its poor adhesion to a substrate and its inability to form a continuous film. As Teflon is insoluble, application of a Teflon film involves spreading a thin film of powdered Teflon onto the surface to be coated, and thereafter the powdered Teflon is sintered in place resulting in either an incomplete film or having many voids. As Teflon is a hard inflexible plastic, a further limitation is that the substrate surface must be rigid otherwise the Teflon will either crack or peel off.
A limited number of commercial fluoropolymers, such as Viton, possess elastomeric properties. However, these materials have relatively high surface energies (as compared to Teflon), poor abrasion resistance and tear strength, and their glass transition temperatures are still high enough (&gt;0.degree. C. for Viton) to significantly limit their use in low temperature environments.
Accordingly there is a need for fluoroelastomers having hydrophobic properties, a surface energies and coefficients of friction at least equivalent to the fluorinated plastics (such as Teflon). Further, such fluoroelastomers must have high adhesion, high abrasion resistance and tear strength, low index of refraction and a low glass transition temperature so that it is suitable for any foreseeably low temperature environment use. Additionally, there is a need for fluoroelastomers that are easily produced in high yields and easy to use. Currently, there are no fluoroelastomers that satisfy all of these needs.
Premonomers
We have discovered and recognized that the conspicuous absence of fluorelastomers in the art exhibiting all of the above enumerated properties can be understood upon analysis of the upstream end of the current processes for synthesis of fluoropolymers and plastics. The kinds and properties of the premonomers currently used in turn result in the limitations in the properties of the monomers, which further limit the diversity and properties of currently known fluoropolymers and fluoroelastomers.
It is known that a haloalkyl oxetane can be substituted in the 3-position with methyl groups containing energetic functional groups such as nitrato, azide, nitro and difluoroamino. The polymerization of these substituted oxetanes in the presence of polyhydroxy aliphatic compounds produces hydroxy-terminated prepolymers having a polyether backbone with pendant energetic groups.
The use of substituted oxetanes as a starting material for the production of polyethers is not new. However, the theme running through the art is that bis-substituted oxetanes are of primary interest and commercial importance. This is understandable in that the bis-haloalkyl oxetane starting material or premonomer is easily produced, whereas the mono-substituted 3-haloalkyl methyl oxetane premonomer is difficult and expensive to produce. There is little teaching in the art for guidance on easy, inexpensive methods of preparation of 3-haloalkyl-3-methyl (mono-substituted) oxetane premonomers or their use in synthesizing mono-substituted fluorinated oxetane monomers.
Bis-haloalkyl oxetane premonomers as a starting material are described in Falk et al. (U.S. Pat. No. 5,097,048). Falk disclose 3,3'-bis perfluoroalkyl oxetane monomers derived from bis-haloalkyl oxetane as a starting material. Reaction of the bis-haloalkyl oxetane with a perfluoroalkyl thiol, a perfluoroalkyl amine, a perfluoroalkanol, or a perfluoroalkyl sulfonamide will produce the 3,3'-bis perfluoroalkyl oxetane monomer described in this reference.
Bis-haloalkyl oxetane premonomers are readily commercially available and their derivatives are fairly well covered in the art. Mono-haloalkyl oxetanes, however, are rarely mentioned in the art, appearing only as an incidental comparison in a more complete investigation of the bis-haloalkyl oxetanes. The lack of teaching regarding the mono-substituted fluorinated alkoxymethylene oxetanes (herein "FOX" compounds for Fluorinated OXetane), and their relative commercial unavailability, is undoubtedly due to the fact that mono-substituted haloalkyl oxetanes are very difficult and expensive to make. Current processes for the production of mono-substituted haloalkyl oxetane premonomers, such as 3-bromomethyl-3-methyloxetane ("BrMMO"), are typified by low yields, long, complicated synthetic schemes and the use of toxic, expensive chemicals to convert 1,1,1-tris(hydroxymethyl)ethane ("TME") into BrMMO.
In these processes, TME is reacted with diethyl carbonate to produce the corresponding cyclic carbonate. This in turn undergoes decarboxylation upon thermal decomposition at 160.degree. C. to provide 3-hydroxymethyl-3-methyloxetane ("HMMO"). The HMMO is converted to the primary chloro compound with carbon tetrachloride and triphenyl phosphine. Reaction of the chloro compound with sodium bromide in methyl ethyl ketone results in S.sub.N 2 displacement of the chlorine to produce BrMMO. This scheme is commercially impractical in that it is both labor intensive and requires expensive, toxic chemicals. Consequently, these disadvantages have precluded the use of mono-substituted fluorinated oxetane (FOX) monomers that may be derived from mono-substituted haloalkyl oxetanes, such as BrMMO, and production of polymer products thereof.
Accordingly, there is a need for a mono-substituted fluorinated alkoxymethylene oxetane monomer with a fluorinated side-chain capable of producing prepolymers and polymers having desirable properties, such as oil and water repellency, at least comparable to the bis-substituted perfluoroalkyl oxetanes known in the literature. Further, there is also a need for a high yielding reaction pathway for production of the mono-substituted haloalkyl premonomer, characterized by a minimum production of by-products, and a commercial feasibility for high volume, high yield production without the excessive labor and materials costs associated with the currently known processes.
Monomers and Prepolymers
The most important criteria in the development of release (i.e., non-stick), high lubricity coatings is the minimization of the free surface energy of the coating. Free surface energy is a measure of the wettability of the coating and defines certain critical properties, such as hydrophobicity and adhesive characteristics of the material. For most polymeric surfaces the surface energy (dispersion component) can be expressed in terms of the critical surface tension of wetting .gamma..sub.C. For example, the surface energy of Teflon (represented by .gamma..sub.C) is 18.5 ergs/cm.sup.2, whereas that of polyethylene is 31 ergs/cm.sup.2. Consequently, coatings derived from Teflon are more hydrophobic and non-stick than those derived from polyethylene. A substantial amount of work has been done by the coating industry to develop coatings with surface energies lower than or comparable to Teflon while at the same time exhibiting superior adhesion characteristics.
The literature teaches that in order to prepare coatings with the desirable low surface energy, the surface of the coating must be dominated by --CF.sub.3 groups. Groups such as --CF.sub.2 --H and --CFH.sub.2 increase the surface energy of the material. The importance of the number of fluorine atoms in the terminal group (i.e., the group present on the surface) was demonstrated in Zisman et al., J. Phys. Chem., 1953, 57, 622; ibid. J. Colloid Sci., 1954, 58, 236; Pittman et al., J. Polymer Sci., 1968, 6, 1729. Materials with terminal --CF.sub.3 groups exhibited surface energies in the neighborhood of 6 ergs/cm.sup.2, whereas similar materials with terminal --CF.sub.2 H groups exhibited values in the neighborhood of 15 ergs/cm.sup.2, more than twice the value for the material with terminal --CF.sub.3 groups. Teflon incorporates the fluorine moieties on the polymer backbone and does not contain pendant --CF.sub.3 groups. Consequently, Teflon does not exhibit surface energies as low as polymers having terminal perfluorinated alkyl side-chains.
A critical requirement in the production of an elastomer is that the elastomer have large zones, or "soft segments", where little or no crosslinking occurs and where the polymer conformation is such that there is little or no compaction of the polymer as a result of crystallization. Intermediate of these soft zones are "hard blocks" wherein there may be significant hydrogen bonding, crosslinking and compaction of the polymer. It is this alternating soft block and hard block which gives the polymer its elastomeric properties. The longer the soft segment, the more elastic the elastomer.
We have discovered that an improved route to producing elastomers is to produce homo- or co-prepolymers characterized as non-cross linked, assymetrical, hydroxy-terminated, linear oligomers having from about 10 to about 500 carbons, preferrably 20 to about 200 carbons. These prepolymers substantially retain their integrity in subsequent polymerizing reactions to provide the soft segment zones of the resulting polymers which, in combination with the hard blocks formed during polymerization, produce good elastomers. We have found that the literature does not have any showing of homo- or co-polymerization of either the bis or the mono-substituted fluorinated alkoxymethylene oxetanes to produce soft segment containing prepolymers required for production of elastomers. Accordingly, there is a need for fluorinated oxetane (FOX) monomers having a side-chain with an omega or terminal perfluorinated alkyl group, which monomers are capable of homo-polymerization or copolymerization to produce the soft segment, herein "FOX prepolymers", necessary for a fluorinated elastomer.
Further, in order for the hydroxy-terminated prepolymer with a fluorinated side-chain (i.e., FOX prepolymers) to be useful, it must have a functionality of at least 2. Presence of non-functional or mono-functional materials in the prepolymers result in coatings with poor mechanical and surface properties. Consequently, these coatings have limited commercial value. Non-functional materials, mainly cyclic tetramers and trimers, are formed during the ring opening polymerization from chain "back-biting". Monofunctional materials, on the other hand are formed due to counter-ion terminations, such as diethyl ether and fluoride ion terminations.
Falk et al. (U.S. Pat. No. 5,097,048) disclose the synthesis of bis-substituted oxetane monomers having perfluoro-terminated alkyl group side chains from bis-haloalkyl oxetane, the glycols having perfluoro-terminated alkyl group side chains derived therefrom, including related thiol and amine linked glycols and dimer diols. Most of the fluorinated side-chains are attached to the glycol unit by either a thio, an amine or a sulfonamide linkage. Only a few of their examples describe glycols with perfluoro-terminated alkoxymethylene side-chains.
Falk et al. (EP 03 48 350) report that their process yields perfluoro-termninated alkoxymethylene neopentyl glycols composed of a mixture of (1) approximately 64% of the bis-substituted perfluoro-terminated alkyl neopentyl glycol, and (2) approximately 36% of a mono-substituted perfluoro-terminated alkyl neopentyl glycol product with a pendant chloromethyl group. Evidently, the mono-substituted product results from incomplete substitution of the second chloride on the bis-chloroalkyl oxetane starting material. Consequently, as noted from the Zisman and Pittman work above, the presence of the --CH.sub.2 Cl as a side-chain significantly increases the surface energy of coatings made from these polymers thus reducing the hydrophobicity and oleophobicity of the coating.
Not surprisingly, it is understandable that Falk et al. (U.S. Pat. No. 5,097,048) discourages the use of the mono-substituted glycol for the preparation of low surface energy coatings, since the monosubstituted glycol as produced from bis-chloroalkyl oxetanes would necessarily have a residual chloromethyl group still attached to the 3-carbon because of the incomplete substitution of the bis-haloalkyl moieties on the starting material. Accordingly, their teaching that the polymer derivatives from mono-substituted glycols do not produce a coating exhibiting the desired properties, to the same extent as coatings derived from bis-substituted glycols is premised on a lower surface energy for the bis-substituted compounds as compared to Falk's mon-substituted compounds (Falk, U.S. Pat. No. 5,097,048, column 1, lines 43-50). However, they ignore the fact that the residual chloromethyl group may serve to increase the free surface energy of the Falk mono-substituted compound more so than the fact it is only mono-substituted in a R.sub.f function.
Moreover, the reference cited by Falk et al. in the '048 patent, J. Org. Chem., 45 (19) 3930 (1980), stating at line 33 that "mono-fluoroalkyl oxetanes containing oxygen have been reported" is misleading in that the reference cited discusses oxetanes substituted with --CH.sub.2 F side chains (i.e., (monofluoro)alkyl oxetanes) and not alkoxymethylene side chains with terminal perfluoroalkyl groups. Hence, this reference will not lead to materials with low surface energies and is not relevant to the compounds of this invention.
Falk et al. (U.S. Pat. No. 5,045,624) teaches preparation of dimers with fluorinated side-chains having thio linkages, but not of dimers with fluorinated ether side-chains. This is because his synthesis route for preparing dimers with thio linkages cannot be used for the synthesis of dimers with ether linkages. In other words, Falk et al. does not teach preparation of long chain polyethers with fluorinated ether side-chains.
Falk et al. (U.S. Pat. No. 4,898,981) teaches incorporation of their bis-substituted glycols into various foams and coatings to impart to them the desired hydrophobicity and oleophobicity. Classic polyurethane chemistry shows that while a plastic may form by reaction of Falk's glycols with the diisocyantes, elastomers can not form since there is no long chain soft segment. As noted above, such a soft segment is needed for the formation of an elastomer. Since the Falk et al. compounds are only one or two monomer units long, it is clearly too short to function as a soft segment for the formation of a polyurethane elastomer. In Falk et al., the fluorinated glycol and isocyanate segments alternate, with the fluorinated glycol segments being nearly the same size as the isocyanate segments. It is well known that such a polymer structure will not yield elastomers.
None of the Falk et al. references teach or show a homo-prepolymer or co-prepolymer made from bis-perfluoro-terminated alkoxymethylene oxetanes, nor polyurethanes derived thereform or from the corresponding glycols. All of their polyurethanes are made directly from the thiol linked monomers and dimers and not via a prepolymer intermediate. In the examples provided in Falk et al. (U.S. Pat. No. 5,045,624), particularly where the perfluoro-terminated side-chains are large and for all of the dimers, all have thiol linkages; no ether side-chains are shown. The polyurethanes disclosed by Falk et al. (U.S. Pat. No. 4,898,981) are made from the perfluoro-terminated alkylthio neopentyl glycol. They do not teach, show or suggest producing a polyurethane from the perfluoro-terminated alkoxy neopentyl glycol monomer, nor do they suggest, teach or show the types of prepolymers and polymers that can be prepared from the mono-substituted 3-perfluoroalkoxymethylene-3-methyl oxetanes (i.e., FOX monomers). However, Falk et al. (U.S. Pat. No. 5,097,048) in their Example 12 show a polyether prepolymer prepared from a bis-substituted perfluoroalkylthio oxetane. The prepolymer obtained was a white waxy solid, clearly not an elastomer. No characterization as to, nature of the end groups, polydispersivity, equivalent weights, etc. of the the waxy solid was given. Absent such a characterization, it is unknown as to whether Falk et al.s' material may be further reacted with an isocyanate to produce a polyurethane polymer. No examples of the preparation of a polymer from any prepolymer is given.
Manser (U.S. Pat. No. 4,393,199) teaches a method for polymerizing oxetane monomers by employing an initiator/catalyst system composed of an alkyl diol and a Lewis acid catalyst, BF.sub.3 etherate. Manser teaches that not all oxetane monomers can be homopolymerized and that the rate of polymerization of bis-substituted oxetane monomers is dependent upon the nature of the substituent at the 3 position on the monomer. Manser does not teach or suggest the polymerization of mono-substituted fluorinated alkoxymethylene oxetanes to produce low viscosity, well defined, difunctional hydroxy-terminated assymetric prepolymers with fluorinated side-chains, nor does he suggest that the prepolymer derived from that polymerization could be cured with diisocyanates to obtain elastomers having exceedingly low surface energies.
Vakhlamova (Chem. Abst. 89:110440p) teaches synthesis of oxetane compounds substituted at the number 3 carbon of the oxetane with --CH.sub.2 O--CH.sub.2 --CF.sub.2 --CF.sub.2 --H groups. The terminal alkyl portion of this substituent is thus: --CF.sub.2 CF.sub.2 --H in which the terminal or omega carbon bears a hydrogen atom. As discussed supra, the Zisman and Pittman works shows that the presence of the hydrogen significantly increases the surface energy of the polymer derived from these monomers. Falk et al. (U.S. Pat. No. 5,097,048) also recognizes that surface energy increases with the hydrogen atom on the terminal carbon by stating that "fluoroalkyl compounds which are terminally branched or contain omega-hydrogen atoms do not exhibit efficient oil repellency". Further, Vakhlamova focuses on the bis-substituted monomer as he hydrolyzes and polymerizes only the bis-substituted monomer.
A characteristic of the polymers formed from the polymerization of the bis-substituted oxetanes of Falk et al., and the other proponents of bis-substituted oxetanes is that the resulting products are crystalline solids. The bis side-chains are highly ordered and symmetric. Consequently, they pack efficiently to form a crystalline structure. For example, a prepolymer prepared from 3,3-bis(chloromethyl)oxetane is a crystalline solid that melts in the neighborhood of 220.degree. C. This significantly affects the commercial use of these polymers as either or both mixing and elevated temperatures will be required in order to dissolve or melt the Falk et al. polymer for further polymerization or application.
Polymerization of the bis-substituted perfluorinated alkoxymethylene oxetanes has received little attention in the art. Moreover, the polymers derived from the bis-substituted perfluoroalkylthiol oxetanes are waxy solids and will not function as a soft segment in the preparation of commercially useful elastomers and coatings. Further, the ability of a bis-substituted oxetane monomer to homopolymerize appears to be dependent upon the nature of the side-chain at the 3 carbon with no assurance such polymerization will occur, the difficulty of polymerization apparently being due to the interference by the 3-carbon side-chains. Polymerization, and the products of polymerization, of the bis monomer accordingly are unpredictable and inconsistent.
Accordingly, there is a need in the art for a fluorinated elastomer product having low surface energies and the other properties enumerated above, and a production strategy therefor, beginning with a premonomer production process that is easy and inexpensive, to produce an assymetrical mono-haloalkyl methyl oxetane premonomer, which upon further reaction produces an oxetane monomer having a single fluorinated side-chain, which mono-substituted fluorinated monomer is capable of homopolymerization and copolymerization to produce an essentially non-cross-linked soft segment, difunctional, linear, assymetric prepolymer for further reaction to produce fluorinated elastomers and thermoset plastics, resins and coatings having hydrophobic properties, low surface energy, very low glass transition temperatures, low di-electric constants, high abrasion resistance and tear strength, high adhesion and low refractive indices.